The present invention relates to a vehicle-mounted radar apparatus which is mounted in an automobile or the like for measuring the relative distance and the relative velocity with respect to a preceding vehicle or the like.
As an apparatus of this type, a vehicle-mounted radar apparatus such as the one shown in FIG. 9 is known. This radar apparatus is comprised of a modulator 1 for outputting a voltage signal for FM modulation; an oscillator 2 for generating an electromagnetic wave subjected to FM modulation; a direction coupler (power divider) 3 for dividing the electromagnetic wave from the oscillator 2 to a transmitting antenna 4 and a mixer 7; the transmitting antenna 4 for outputting the electromagnetic wave to space in such as the forward direction of the vehicle; a receiving antenna 6 for receiving the electromagnetic wave reflected from a target object 5; the mixer 7 for mixing the transmitted electromagnetic wave and the received electromagnetic wave; and a signal processing circuit 8 for computing the relative distance to the target object and the relative velocity on the basis of a beat signal outputted from the mixer 7.
Next, a description will be given of the operation of the conventional apparatus configured as described above. The modulator 1 outputs a voltage signal for linear FM modulation. The voltage controlled oscillator 2 generates an electromagnetic wave subjected to FM modulation on the basis of the voltage signal for FM modulation. The electromagnetic wave is divided into two portions by the power divider 3, and one portion is inputted to the mixer 7. The other portion is outputted from the transmitting antenna 4 into space. The electromagnetic wave outputted to space from the transmitting antenna 4 is reflected by the target object 5, and is inputted to the receiving antenna 6 with a delay time Td with respect to the transmitted electromagnetic wave. Further, in a case where the target object 5 has a relative velocity, the received electromagnetic wave is inputted to the receiving antenna 6 with a Doppler shift Fd with respect to the transmitted electromagnetic wave. The electromagnetic wave received by the receiving antenna 6 is mixed with the transmitted electromagnetic wave by the mixer 7, and a beat signal corresponding to the time delay Td and the Doppler shift Fd is outputted. On the basis of the beat signal, a signal processor 9 computes the relative distance to the target object 5 and the relative velocity.
Next, a description will be given of a method of computing the relative distance and the relative velocity. FIG. 10 shows one example for computing the relative distance and the relative velocity using the above-described radar apparatus. In FIG. 10, the transmitted electromagnetic wave is subjected to FM modulation with a frequency sweep bandwidth B and a modulation period Tm of the transmitted electromagnetic wave. The received electromagnetic wave waits for the time delay Td from the time the transmitted electromagnetic wave is reflected by the target object 5 which is present at a distance R until it is inputted to the receiving antenna 6. In addition, in a case where the target object 5 has a relative velocity, the received electromagnetic wave undergoes a Doppler shift by Fd with respect to the transmitted electromagnetic wave. At this time, a frequency difference Fbu between the transmitted signal and the received signal during a frequency rise and a frequency difference Fbd between the transmitted signal and the received signal during a frequency drop are outputted as the beat signal by the mixer. On the basis of the frequency differences Fbu and Fbd, the modulation period Tm, and the frequency sweep bandwidth B, as well as the light velocity C (=3.0.times.10.sup.8 m/s) and the wavelength .lambda. of a carrier wave (if the fundamental frequency F.sub.o of the carrier wave is 60 GHz, then .lambda.=5.0.times.10.sup.-3 m, the relative distance R and the relative velocity V with respect to the target object can be determined by the following formulae (1): EQU R=Tm.multidot.C/8B(Fbu+Fbd),V=.lambda./4(Fbu-Fbd) (1)
In addition, in a case where a plurality of target objects 5 are present, Fbu and Fbd of the same object are selected from a plurality of frequency differences Fbu between the transmitted signal and the received signal during a frequency rise and a plurality of frequency differences Fbd between the transmitted signal and the received signal during a frequency drop, and the relative distance R and the relative velocity V are determined in accordance with the formulae (1).
Next, if it is assumed that the resolutions (minimum steps of data values outputted discretely) of the relative distance R and the relative velocity V are .DELTA.R and .DELTA.V, respectively, the resolutions .DELTA.R and .DELTA.V can be respectively determined by the following formulae (2): EQU .DELTA.R=C/2B,.DELTA.V=.lambda./Tm (2)
The above-described radar apparatus is used for a following-distance warning apparatus which informs the driver of a danger by issuing a warning when a distance with a preceding vehicle has become shorter than a safe following distance and the danger of a collision has become heightened, or for effecting follow-up traveling by keeping a safe following distance. However, particularly when follow-up traveling is effected, if the aforementioned modulation period Tm is short, the resolution .DELTA.V of the relative velocity V deteriorates, so that the riding comfort becomes aggravated. In addition, if the modulation period Tm is long, the data output period becomes long, with the result that the position and the velocity of the preceding vehicle change substantially during the output period, resulting in the deterioration of measurement accuracy.
To overcome the above-described problems, in Unexamined Japanese Patent Publication 8-211145/(1996) and 8-5733/(1996), the modulation period Tm is changed in correspondence with the traveling conditions, and the modulation period Tm is repeatedly changed to measure with high accuracy objects including those located in the far distance and those located in the near distance.
In a case where, for example, the above-described vehicle-mounted radar apparatus is made to continuously effect scanning mechanically, if the modulation period Tm is made long, the velocity resolution .DELTA.V improves. However, the amounts of change of the relative distance and the relative velocity of the target object become large, so that the accuracy of the measured relative distance and relative velocity declines, and there are cases where-the object which reflects the electromagnetic wave during a transmission frequency rise and the object which reflects the electromagnetic wave during a transmission frequency drop are not identical, outputting erroneous data.